Diesel engines have relatively high efficiency as compared to other internal combustion engines. With the higher engine efficiency comes lower engine exhaust temperatures. Lower exhaust temperatures can make regeneration of after treatment devices more difficult because after treatment devices often begin to regenerate at higher temperatures. One way to produce higher temperatures in an exhaust system is to inject fuel late in a cylinder cycle (e.g., during an exhaust stroke) so that the fuel can oxidize in the exhaust system, thereby increasing the exhaust system temperature. However, when liquid fuel is injected in an exhaust stroke, some portion of the injected fuel can impinge on cylinder walls in liquid form. Fuel that encounters cylinder walls in liquid form may degrade the oil film on the cylinder wall and increase cylinder wall wear. Further, liquid fuel may enter the engine crankcase and dilute engine oil. Consequently, it may be desirable to reduce the amount of injected liquid fuel that reaches cylinder walls during injection.
In European patent application EP 1,798,404 A1, a method for regenerating an after treatment device via post injection control is described. The method estimates a fuel spray brakeup length based on a differential pressure between an interior and exterior of an injector hole. The method adjusts fuel injection time so that the fuel spray brakeup length is less than a distance S to a cylinder wall. However, the fuel spray brakeup length as determined via a delta pressure may not be as accurate as is desired. Consequently, fuel spray may still impinge on cylinder walls during some conditions.
The inventor herein has recognized the above-mentioned disadvantages and have developed a method for regenerating an exhaust after treatment device, comprising: performing combustion in a cylinder of an engine during a cylinder cycle; injecting an amount of fuel in a fuel injection pulse after a combustion event in the cylinder and before exhaust valve closing during the cylinder cycle, the amount of fuel in the fuel injection pulse adjusted for a density of a gas mixture in the cylinder; and regenerating an after treatment device via the amount of fuel.
By adjusting a post injection fuel quantity in response to cylinder mixture density, it may be possible to better estimate an amount of fuel injected that impinges on a cylinder wall in liquid form so that fuel injected for after treatment device regeneration is less than the amount that impinges on the cylinder wall in liquid form when injected. In particular, an estimate of fuel spray penetration may be improved by accounting for cylinder density and temperature. Cylinder mixture density and temperature provide a more accurate estimate of fuel spray penetration than cylinder pressure or injector delta pressure since cylinder mixture density and temperature account for both fuel evaporation and momentum transfer.
Additionally, the inventor has also determined that cylinder volumetric efficiency is another parameter that may be accounted for to improve fuel spray penetration estimates. Cylinder volumetric efficiency can affect temperature of gases in a cylinder as well as fractions of air and residual exhaust gas that make up a cylinder mixture. In this way, cylinder volumetric efficiency can affect density and temperature within the cylinder at the time of injection. Consequently, by considering cylinder mixture density, cylinder volumetric efficiency, and temperature, timing of a post injection fuel amount may be improved so that fuel spray more closely approaches a cylinder wall, yet does not impinge on the cylinder wall in liquid form. As a result, larger amounts of fuel may be injected into a cylinder without causing fuel to impinge on cylinder walls in liquid form.
The present description may provide several advantages. In particular, the approach may reduce engine degradation. Further, the present approach may reduce engine emissions by reducing engine oil dilution. Further still, the approach may be applied without costly in-cylinder sensors. Additionally, the approach may increase an engine oil change interval and/or allow for a smaller oil sump containing less oil.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.